1. Field of the Invention
The present invention relates to a photo-mask used for a semiconductor device and a manufacturing method thereof, and a registration accuracy enhancement method thereof.
2. Description of the Background Art
A registration accuracy measurement mark has conventionally been used for measuring registration accuracy of each layer in a semiconductor device manufacturing process. The registration accuracy measurement mark is usually formed in several layers, that is, in each of the stacked inherent layers of the device which are the object of actual registration accuracy measurement, at a predetermined position on a semiconductor substrate. When measuring registration accuracy, the positions of stepped portions, i.e. edges of the registration accuracy measurement mark are detected, and the distance between the two detected stepped portions, i.e., edges is measured, whereby the amount of displacement of the registration between the registration accuracy measurement marks in two layers is calculated. The difference between the amount of displacement of the registration of the two opening patterns in the inherent layers formed on a photo-mask and the calculated amount of displacement of the two stepped portions of the registration accuracy measurement marks is thereby measured as a registration error in a manufacturing process.
Though the above-described stepped portions of the registration accuracy measurement mark are detected using a registration accuracy measurement device, the registration error itself cannot be accurately measured because the displacement of the pattern, i.e., transfer error, caused by lens aberration is not considered.
A method of measuring registration accuracy considering a lens aberration is described in Japanese Patent Laying-Open No. 9-74063. In the technology described in the laid-open application, registration accuracy measurement marks are formed in two layers in the same shape as two inherent layers to be measured for actual registration accuracy, so that the amount of the pattern displacement, i.e., transfer error, of the two inherent layers due to aberration and that of the registration accuracy measurement marks due to aberration come to be the same. The pattern displacement of the inherent patterns caused by aberration and that of the registration accuracy measurement marks caused by aberration are thereby offset. Therefore, only the registration error is detected in the registration accuracy measurement method described in Japanese Patent Laying-Open No. 9-74063.
In a registration accuracy measurement method using only a conventionally used registration accuracy measurement mark, however, the pattern displacement itself caused by lens aberration is not calculated to modify a lens unit (hereinafter, the modification of a lens unit means both the adjustment of the lens unit and the correction of a lens) so as to reduce the aberration. Thus, the registration error caused by lens aberration is not corrected to enhance the registration accuracy in a semiconductor device manufacturing process.
The following method, however, is generally used to evaluate the amount of aberration. First, a resist film 100, provided with a pattern in which lines and spaces are alternately formed, is formed as shown in FIGS. 7 and 8. The line-widths of L1 and L2 at the opposite ends of resist film 100 are measured, for example, using SEM (Scanning Electron Microscope). The line-width abnormal value due to comatic aberration is generally calculated by the following equation:
Line-width abnormal value=(L1xe2x88x92L2)/(L1+L2)xe2x80x83xe2x80x83(1)
From the equation (1), the line-width abnormal value can be calculated according to the measurement results of the above-described widths L1 and L2.
In this method, only a pattern in which lines and spaces are alternately formed is used for resist film 100, so that only the measurement error of the pattern line-width caused by aberration can be detected when using normal SEM. That is, a pattern transfer error resulting from the difference in shape between two patterns to be measured due to the aberration can not be detected. Thus, in manufacturing a semiconductor device, a lens unit which is modified to reduce the transfer error of the pattern caused by lens aberration has not been used.
Furthermore, since SEM is used, the measurement time is long and the measurement of the whole lens unit (the entire exposure device) is difficult.
One object of the present invention is to provide a semiconductor device with registration accuracy enhanced, a photo-mask used for manufacturing the device, and a registration accuracy enhancement method thereof, by detecting lens aberration involved in a semiconductor manufacturing process.
Another object of the present invention is to enable correction of lens aberration by using a normal registration accuracy measurement device.
A semiconductor device according to the invention includes an auxiliary mark for registration accuracy enhancement (hereinafter simply referred to as auxiliary mark) used-to detect aberration of a lens or lenses used in an exposure step in the manufacturing of a semiconductor device so as to modify the lens unit to reduce the lens aberration. The auxiliary mark includes an inner mark forming four sides of a first virtual rectangle when viewed two-dimensionally and an outer mark forming four sides of a second virtual rectangle analogous to the first virtual rectangle and having the same intersection point of diagonals as the first virtual rectangle when viewed two-dimensionally. The inner and the outer marks are formed to have stepped portions, which stepped portions belong to one same layer and can be detected by a registration accuracy measurement device.
Such a structure allows the auxiliary mark to have the outer mark on the same layer as the inner mark. Thus, in the exposure step using a photo-mask for forming the auxiliary mark, the inner and outer marks can be formed not affected by a registration error caused by overlaying two layers.
Therefore, when the patterns of the inner and outer marks formed on a semiconductor substrate are compared with those opened in the photo-mask, an error in the distance between stepped portions of the respective inner and outer marks will approximately be the same as a pattern transfer error caused by lens aberration in the exposure step using the photo-mask, since an alignment error caused when using an exposure device such as a stepper is sufficiently small.
As a result, lens aberration due to the pattern difference between the inner and outer marks can be detected at a position where the auxiliary mark is provided, by using a normal registration accuracy measurement device. By modifying the lens unit based on the detected result to reduce the lens aberration, the registration accuracy can be enhanced for each layer to be positioned one upon another.
Further, in a semiconductor device according to the invention, the outer mark having the stepped portions may be formed as a box pattern, a line pattern, or a hole pattern.
In a semiconductor device according to the invention, the inner mark having the stepped portions may be formed as a box pattern, a line pattern, or a hole pattern.
Alternatively, in a semiconductor device according to the invention, the outer mark having the stepped portions may be formed as a positive pattern (i.e., a pattern on which the resist remains) or a negative pattern (i.e., a pattern from which the resist is eliminated).
In a semiconductor device according to the invention, the inner mark having stepped portions may also be formed as a positive pattern or a negative pattern.
Alternatively, in a semiconductor device according to the invention, the auxiliary mark may preferably be formed to be a plurality of marks dispersed across the entire exposure region on a semiconductor substrate.
Such a structure in that auxiliary marks are dispersed on the semiconductor substrate allows detection of a pattern transfer error caused by the lens aberrations at respective positions. This leads to a more accurate enhancement of the registration accuracy for each layer placed one upon another in a semiconductor device manufacturing step, as the lens unit is modified to reduce the lens aberrations at respective positions.
In a semiconductor device according to the invention, more preferably, a plurality inner marks are formed to have a number of steps having different sizes.
Such a structure enables detection of the aberration depending on the sizes of inner marks, as a plurality of inner marks having different sizes are dispersed on a semiconductor substrate. As a result, registration accuracy for each overlaid layer can accurately be enhanced in the process of manufacturing a semiconductor device, as the lens unit is modified to reduce the lens aberration for the different sizes of the inner marks.
In a semiconductor device according to the invention, more preferably, a plurality of outer marks are formed to have a number of steps having different sizes.
Such a structure enables detection of the aberration depending on the sizes of outer marks, as a plurality of outer marks having different sizes are dispersed on a semiconductor substrate. As a result, registration accuracy for each overlaid layer can accurately be enhanced in the process of manufacturing a semiconductor device, as the lens unit is modified to reduce the lens aberration for the different sizes of the outer marks.
In a semiconductor device according to the invention, more preferably, the plurality of inner marks are formed to include a box pattern, a line pattern and a hole pattern.
Such a structure enables detection of the aberration that varies depending on the pattern shapes, as the plurality of auxiliary marks, in which the inner marks include a box pattern, a line pattern, and a hole pattern, are dispersed on a semiconductor substrate. As a result, registration accuracy for each layer can accurately be enhanced in the process of manufacturing a semiconductor device, by modifying a lens unit to reduce the lens aberration for each pattern shape.
Alternatively, in a semiconductor device according to the invention, the plurality of outer marks are formed to include a box pattern, a line pattern and a hole pattern.
Such a structure enables detection of the aberration that varies depending on the pattern shapes, as the plurality of auxiliary marks, in which the outer marks include a box pattern, a line pattern, and a hole pattern, are dispersed on a semiconductor substrate. As a result, registration accuracy for each layer can accurately be enhanced in the process of manufacturing a semiconductor device, by modifying a lens unit to reduce the lens aberration for each pattern shape.
A photo-mask for manufacturing a semiconductor device according to the present invention is provided with an opening corresponding to the pattern of the auxiliary mark which includes an inner mark forming four sides of a first virtual rectangle when viewed two-dimensionally and an outer mark forming four sides of a second virtual rectangle analogous to the first virtual rectangle and having the same intersection point of diagonals as the first virtual rectangle when viewed two-dimensionally, the inner and outer marks having stepped portions on one same layer, which can be detected by a registration accuracy measurement device.
By using such a photo-mask, a semiconductor device according to the above-described aspects of the present invention can be manufactured.
A method of enhancing the registration accuracy of a semiconductor device according to the invention uses the auxiliary mark which includes an inner mark forming four sides of a first virtual rectangle on a semiconductor substrate when viewed two-dimensionally and an outer mark forming four sides of a second virtual rectangle analogous to the first virtual rectangle and having the same intersecting point of diagonals as the first virtual rectangle when viewed two-dimensionally, the inner and outer marks having stepped portions on one same layer, which can be detected by a registration accuracy measurement device.
The detection of lens aberration by such a method allows the lens unit to be modified to reduce the lens aberration, so that a semiconductor device with an enhanced registration accuracy for each layer placed one upon another can be manufactured.